Portable battery powered wireless communications devices, such as mobile terminals, cell phones, and the like, often have requirements to efficiently transmit information at different output power levels. As a result, RF transmitter power amplifiers need to transmit over a wide range of output power levels, while maintaining efficiency throughout the operating range. Traditional power amplifier designs may operate over a wide range of output power levels, but only achieve maximum efficiency at or near maximum output power. One approach for efficiently changing output power levels is incorporation of a dual mode power amplifier, which operates in either a high power output mode or a low power output. Dual mode power amplifiers may operate relatively efficiently in either mode; however, these dual mode power amplifiers require complex architectures and may have difficulty meeting linearity requirements.
Further, the output powers of single-path power amplifier designs may change with changing antenna loading conditions. The load sensitivity of a single-path amplifier is especially problematic at high power levels where the amplifier may not have the capability to maintain the desired output power into a mismatched load, or the amplifier may consume excessive current with a mismatched load, or both. The output powers of quadrature power amplifier designs tend to remain stable in the presence of changing antenna loading conditions. Therefore, there is a need for a dual mode quadrature RF power amplifier with a simple architecture that operates efficiently in both modes.